A Forward Unprovability Calculus for Intuitionistic Propositional Logic

The inverse method is a saturation based theorem proving technique; it relies on a forward proof-search strategy and can be applied to cut-free calculi enjoying the subformula property. This method has been successfully applied to a variety of logics. Here we apply this method to derive the unprovability of a goal formula G in Intuitionistic Propositional Logic. To this aim we design a forward calculus FRJ(G) for Intuitionistic unprovability. From a derivation of G in FRJ(G) we can extract a Kripke countermodel for G. Since in forward methods sequents are not duplicated, the generated countermodels do not contain redundant worlds and are in general very concise.